The invention relates to thermoplastic C.sub.2 -.alpha.-olefin copolymer resin blends and flexible films thereof having heat sealing and puncture resistance properties. Such blends are useful for making films particularly heat shrinkable, oriented films for packaging food and non-food articles, but especially fresh or frozen foods such as meat, poultry or cheese.
Manufacturers and wholesalers utilize flexible thermoplastic packaging films to provide economical, sanitary containers which help protect and/or preserve the freshness and wholesomeness of their products. These films are often sold in bag form. For example, a single or multilayer thermoplastic film may be made into bags by a packaging manufacturer using film stock comprising a tubular film or one or more flat sheets or webs of film by well known processes involving e.g. cutting, folding and/or sealing the film to form bags which may then be shipped to processors for use in packaging operations. These films and bags may be printed with customer logos, product data or other information and may also be uniaxially or biaxially oriented, heat shrinkable, or irradiated, or may contain film layers which are abuse resistant or puncture resistant, or which are crosslinked or which enhance or retard or prevent transmission of light, gases, or liquids therethrough. Frequently, multilayer films having one or more barrier layers to oxygen and/or moisture such as saran (a polyvinylidene chloride copolymer), a modified saran e.g. containing methyl acrylate polymer units, ethylene vinyl alcohol copolymer, nylon, or acrylonitrile may be used with a heat sealing layer such as a copolymer of ethylene and vinyl acetate (EVA) to produce bags for packaging oxygen and/or moisture sensitive foods e.g. fresh red meat. Such bags help preserve meat in its original condition by preventing or reducing moisture loss and chemical changes in the meat structure due to oxidation reactions. A typical packaging bag produced from a tubular film stock will have one or two sides which have been heat sealed by the bag manufacturer in the bag forming process. For food packaging bags often will have one open side to allow a food processor to insert ham, turkey, chicken, cheese, primal or subprimal meat cuts, ground beef, fruits, vegetables, bread or other food products into the bag. The food processor then makes a final seal thereby enclosing the bag. This final seal may follow gas evacuation of the bag by vacuum means or replacement of the gaseous environment within the bag by a particular gas or mixture of gases which may be inert or reactive with the enclosed product to provide some advantage such as to assist product preservation. This final seal is frequently a heat seal similar to the initial seals produced by the bag manufacturer although the actual heat sealing equipment may vary.
Thus, commercially available bags are made by transversely sealing a tubular stock of either monolayer or multilayer film and cutting off the tube portion containing the sealed end, or by making two spaced apart transverse seals on a tubular stock and cutting open the side of the tube, or by superimposing flat sheets of film and sealing on three sides, or by end folding flat sheets and sealing two sides.
Generally heat sealing of thermoplastic film is accomplished by applying sufficient heat and pressure to adjacent film layer surfaces for a sufficient time to cause a fusion bond between the layers.
A common type of seal used in manufacturing bags is known to those skilled in the art as a hot bar seal. In making a hot bar seal, adjacent thermoplastic layers are held together by opposing bars of which at least one is heated to cause the adjacent thermoplastic layers to fusion bond by application of heat and pressure across the area to be sealed. For example, bags may be manufactured from a tube stock by making one hot bar seal transverse to the tube. This seal may also be referred to as a bottom seal. Once the bottom seal is applied, the tube stock may be transversely cut to form the mouth of the bag.
The strength of seals of heat shrinkable bags may be measured by determining the time for a seal to fail when under certain conditions the seal is immersed in hot water e.g. at 95.degree. C. i.e., the hot water seal strength ("HWSS") may be measured by a test such as that described as the "restrained shrinkage-seal strength test" in Funderburk et al U.S. Pat. No. 3,900,635 which patent is hereby incorporated by reference.
Once a product such as meat or poultry is inserted into the bag, the package is typically evacuated and the bag mouth sealed. At one time, the standard method for sealing a bag was to fasten a clip around the mouth of the bag. However, heat sealing techniques are now also commonly employed to produce the final closure of the bag. For example, a bag mouth may be hot bar sealed or it may be sealed by another common type of heat seal known as an impulse seal. An impulse seal is made by application of heat and pressure using opposing bars similar to the hot bar seal except that at least one of these bars has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name "impulse") to cause the adjacent film layers to fusion bond. Following the impulse of heat the bars are cooled (e.g. by circulating coolant) while continuing to hold the bag inner surfaces together to achieve adequate sealing strength.
Generally, impulse seals may be made faster than hot bar seals because of the quick cool down of the impulse ribbon following the heat impulse. Impulse seals are also generally narrower than hot bar seals which lead to an improved package appearance, but narrower seals also leave less margin for error in the production of continuous sealed edges. Since typically less area is bonded in an impulse seal relative to a hot bar seal, the performance of the sealing layer of the thermoplastic film is more critical.
One problem which occurs during impulse heat sealing of known films is that the film in the seal area often becomes extruded during sealing. This results in thinning of the film in the seal area and therefore reduces the strength of the film at the seal or in extreme situations, allows the thinned film to be too easily severed or pulled apart. Those skilled in the art refer to severely extruded seals as "burn through" seals. Thus, a "burn through" seal does not have adequate strength or integrity to seal in or protect the packaged product. One attempt to solve this "burn through" problem is to irradiate the film prior to manufacture of the bag.
Irradiation of a multilayer film made from cross-linkable polymer resins causes the various irradiated resin layers in the film to crosslink. Under controlled conditions, crosslinking by irradiation raises and may also broaden the temperature range for heat sealing, and depending upon the film composition may also enhance puncture resistance of the film. If the heat sealing layer of the thermoplastic film is crosslinked too heavily, the highly crosslinked layer is more difficult to melt or fusion bond which makes it difficult to achieve strong seals, particularly by impulse sealing the bag mouths after filling with meat or poultry. All of the bag seals (including those made by both the bag manufacturers and the food processor and by whatever means including either or both hot bar seals and impulse seals must maintain their integrity to preserve and protect the enclosed food product.
There must be a strong continuous seal to prevent unwanted egress and ingress of gaseous, liquid or solid materials between the bag exterior and interior. This is particularly necessary when the package is made of heat shrinkable film and is to be immersed in hot water to shrink the film against the packaged article since such shrinkage increases the stress on these seals. Thus, there is a continuing need for monolayer and multilayer films which can be made into bags having strong seals especially when formed by hot bar sealing or impulse sealing. Such films should provide strong seals able to withstand a range of temperatures and also be able to produce such seals over a wide sealing temperature range without burn through.
Variations in sealing temperatures, times and pressure are known to exist not only from one brand and/or type of sealers to another but also between different sealing machines sold by the same manufacturer under the same brand identification. Such variations, which may be due to factors such as variation in the manufacturer's product or varying equipment settings or installation, increase the desirability for films which may be heat sealed to produce strong integral seals over a wide range of temperatures and therefore be usefully sealed on different sealing machines.
Another problem encountered during heat sealing is that of inadvertent folding. Normally, a heat seal is made by applying heat and pressure across two sheets or portions of film e.g. the two opposing sides of a flattened tube, however, occasionally the area to be sealed will be inadvertently folded to produce a section of film having four or six sheets or film portions which are pressed between the opposing sealer bars. In such situations it is desirable to be able to seal the film without burn through. A wider impulse heat sealing temperature range is indicative of a greater latitude in sealing through folds than a narrower range.
Copolymers of ethylene and vinyl esters such as vinyl acetate have previously been disclosed as useful materials in monolayer and multilayer thermoplastic films and are known for providing heat sealing properties.
For example, U.S. patent application Ser. No. 07/419,061 (Georgelos et al) filed Oct. 10, 1989, which application is hereby incorporated by reference,disclose EVA blends useful for their heat sealing properties.
U.S. Pat. No. 4,064,296 (Bornstein et al) discloses a heat shrinkable multilayer film having an oxygen barrier core layer of hydrolyzed ethylene-vinyl acetate (EVOH) and outer layers of EVA.
U.S. Pat. No. 4,178,401 (Weinberg et al) discloses an oriented, heat shrinkable packaging film having a blended self-welding layer said to have superior seal strength and abuse resistance. Blends of EVAs with different melt flows are disclosed with e.g. a first EVA having a melt flow of less than 5.0 blended with a second EVA having a melt flow of at least 28. The film may also be crosslinked by irradiation.
U.S. Pat. No. 4,247,584 (Widiger et al) discloses heat sealable food bags made from multilayer films having a heat sealing layer comprising a blend of EVAs with 10 to 90 weight percent of the blend comprising a first EVA having 2-12% VA and a melt index of 0.2 to 10 dg/min. and 90 to 10 weight percent of the blend comprising a second EVA having 8-30% VA and a melt index of 0.2 to 5.
An example of a typical fresh red meat bag currently in commerce is a film having three layers which are coextruded and oriented. The core or middle layer of the film is an oxygen and moisture barrier material, the outer layer provides abrasion resistance and is formulated to provide support for the film during the expansion of the primary tube for orientation, and the inner layer provides heat seal properties and contributes to puncture resistance.
The core or barrier layer of this film is a relatively small percentage of total film thickness and is made of polyvinylidene chloride (PVDC) or vinylidene chloride methylacrylate copolymer (VDC-MA or MA-Saran).
The outer layer of this film is thicker than the core layer and is a blend of very low density polyethylene (VLDPE) and ethylene vinyl acetate (EVA). VLDPE, also called ultra low density polyethylene is a class of ethylene-alpha olefin copolymers having a density which generally is recognized to range from less than 0.915 g/cm.sup.3 down to about 0.860 g/cm.sup.3. The EVA and VLDPE components contribute to the shrink properties of the film and the VLDPE component contributes to the abrasion and puncture resistance. The VLDPE also adds plastic orientation strength to minimize breaks of the secondary bubble during expansion of the softened primary tube.
By far, the thickest film layer is the inner or heat seal layer. In the commercial film noted above, this layer is over 60% of the total film thickness and comprises a blend of VLDPE and EVA. The heat seal layer provides a significant contribution to the puncture resistance properties of the film. Another desirable film characteristic provided by this layer is the heat seal temperature range. It is preferred that the temperature range for heat sealing the film be as broad as possible. This allows variation in the operation of the heat sealing equipment as opposed to a film having a very narrow heat sealing range. For example, it is desirable for a suitable film to heat seal over a temperature range of 350.degree. F. to 550.degree. F., providing a heat sealing window of 200.degree. F.
While films of the general structure and composition as described above have been in commercial use for many years, efforts continue to improve upon such films and in particular to increase puncture resistance while maintaining ease of processability, a broad heat seal temperature range and a high degree of both machine direction (MD) and transverse direction (TD) shrink.
Recent developments for improving properties of a heat shrinkable film include U.S. Pat. No. 5,272,016 (Ralph). The '016 Patent improves properties of a multilayer nonoxygen barrier film by forming the outer layers of a blend of EVA, VLDPE and a plastomer.
U.S. Pat. No. 5,397,640 discloses a multilayer oxygen barrier film wherein at least one outer film layer is a three component blend of VLDPE, EVA and a plastomer.
U.S. Pat. No. 5,403,668 discloses a multilayer heat shrinkable oxygen barrier film wherein one of the film outer layer is a four component blend of VLDPE, LLDPE, EVA and plastomer.
Recent manufacturing changes in catalysts and processes have provided increasing numbers of polymeric resins having different melting characteristics and melting points, and narrower molecular weight distribution ratios (MWD). MWD is the ratio of M.sub.w /M.sub.n, where M.sub.w is the weight-average molecular weight of the resin and M.sub.n is the number-average molecular weight. For example, older commercialized VLDPE resins have a MWD generally in the range of about 3.5 to 8.0, although some VLDPE resins outside this range have been commercialized. Improvements in catalysis technology has been able to produce many resins which reduce this ratio generally to below 3 to the range of about 1.5 to about 2.5 and most typically to about 2.0. This reduction in the MWD means that the polymer chains of these VLDPE resins are more uniform in length. Whereas those having a higher MWD may be said to comprise polymer chains of more varied lengths. Other differences in resin properties have been attributed to differences in distribution along an ethylene backbone resulting in materials produced from single-site catalysts having a lower melting point than a multisite catalyst produced VLDPE of comparable density and melt index. Also, in the case of the above-noted commercial film wherein the heat seal layer is primarily a blend of EVA and VLDPE, it was found that using a more narrow M.sub.w /M.sub.n VLDPE having a lower melting point in place of a broader M.sub.w /M.sub.n VLDPE having a higher melting point considerably decreased the operable heat sealing range. For example, where the sealing layer used only a very narrow M.sub.w /M.sub.n, lower melting point VLDPE in the blend, the heat seal temperature was in the order of 400.degree. F. to about 475.degree. F. giving a sealing window of only 75.degree. F.
Past attempts at providing improved heat sealing in films, while making some progress, leave much to be desired. Variability in heat sealing equipment and process parameters continue to produce bags with weak seals which are subject to burn through, which fail to seal through folds, and which produce leaking bags having discontinuous seals. It would be highly desirable to have biaxially stretched, heat shrinkable films and bags whose heat sealing layer in particular and film construction in general allows greater flexibility and variability in heat sealing process parameters while producing strong, integral, continuous seals rapidly and with a lower failure rate relative to prior art films and bags.
Accordingly, one object of the present invention is to provide a novel polymeric blend having improved heat sealing properties.
Another object of the invention is to provide a polymer blend having an improved combination of properties.
Another object of the invention is to provide a flexible film having improved heat sealing properties.
Another object of the present invention is to provide a heat shrinkable biaxially oriented monolayer film having improved puncture resistance and/or a broad heat sealing range.
Another object of the invention is to provide a heat shrinkable biaxially oriented multilayer film having a broad heat sealing range.
Another object of the invention is to provide a heat shrinkable biaxially oriented multilayer film having improved puncture resistance.
Another object of the present invention is to provide a heat shrinkable biaxially oriented multilayer film having an improved combination of puncture resistance and a broad heat sealing range.
Yet another object of the present invention is to provide a heat shrinkable, biaxially oriented multialyer film having a puncture resistance and heat sealing range suitable for use in the packaging of fresh bone-in meats.
A still further object of the present invention is to provide a heat shrinkable, biaxially oriented multilayer film having an improved combination of optical properties, heat sealing properties and puncture and abrasion resistance.